Primidone

How it works

It is metabolised to phenobarbital and phenylethylmalonamide, which enhance GABA-mediated inhibition and reduce neuronal excitability to raise the seizure threshold and dampen tremor.

Prescribing in practice

• It is a potent hepatic enzyme inducer that lowers the levels of many co-prescribed drugs, including hormonal contraceptives and anticoagulants, and it must not be stopped abruptly because of the risk of seizures.
• Sedation, dizziness and ataxia are common, particularly at the start, so it is introduced at a low dose and titrated slowly.
• It carries the teratogenic and dependence risks associated with barbiturates and requires care in older and frail patients.

Monitoring

Monitor for sedation, ataxia and mood change, with full blood count and seizure or tremor control, and be alert to enzyme-induction interactions.

Counselling the patient

• Never stop this medicine suddenly, as doing so can trigger seizures.
• It often causes drowsiness and unsteadiness when first started, which usually eases; avoid driving until stable.
• It can make the contraceptive pill and some other medicines less effective, so seek advice on reliable contraception.

Evidence & guidelines

Primidone is an established treatment for epilepsy and is recognised as a first-line option for essential tremor in clinical guidance, with efficacy supported by long-standing trial evidence.